The present invention relates generally to digitizing tablet systems and more particularly to a digitizing tablet system which is constructed so as to automatically correct for stylus tilt errors.
Digitizing tablet systems are well known in the art and are used in a variety of applications. These systems generally include a tablet, a stylus and some instrumentality for producing some form of interaction between the stylus and the tablet from which is derived digital data signals representing the position of the stylus on the tablet.
In a number of these systems the tablet contains a grid of conductive elements and the stylus contains an electric coil. Inductive type of interaction between the coil in the stylus and the grid in the tablet is achieved by energizing either the coil or the grid with an AC voltage signal and then measuring the voltage signal induced in the other element. In other systems, capacative type coupling with the grid in the tablet is achieved by using a flat conductive disk in the stylus in place of the coil. In addition to the conductive element, the stylus usually contains either a ball point pen or a pencil with the tip of the pen or pencil terminating at the tip of the stylus so that the stylus can write or draw on a surface as its position is being monitored. Since the pen or pencil point is at the tip of the stylus, the conductive element is, by necessity, and because of its physical size located within the stylus barrel at some finite distance, such as one half of an inch, from the tip.
In some digitizing tablet sysems the stylus is held in the hand of the user while in other digitizing tablet systems the stylus is mechanically mounted at right angles to the working surface of the tablet.
When the stylus is held in the hand of the user it is generally not held at right angles to the tablet but rather at some acute angle thereto. Furthermore, in the course of writing on the tablet the angle will very often change. In systems in which the element in the stylus which interacts with the element in the tablet is located at the exact tip of the stylus and directly contacts an element in the tablet, the angle in which the stylus is held relative to the tablet will generally not be a factor. However, if the element is not at the tip of the stylus an error, called stylus tilt error, will occur if the stylus is not exactly perpendicular to the working surface of the tablet. This will happen because the position signal produced by the location of the conductive element in the stylus relative to the grid of conductive elements in the tablet will not correspond the position where the tip of the stylus is actually in contact with the tablet. Furthermore, if the angle of tilt changes as a line or character is drawn, which is very often the case, the size of the tilt error from point to point over the line or character will change.
For many applications stylus tilt error is not acceptable.
In U.S. Pat. No. 3,873,770 to J. Ioannou there is disclosed a digitizing tablet system in which tilt error is described as being compensated for by a complex technique which involves measuring changes in the positive and negative peaks of the envelope waveform produced when the array of conductors are sequentially energized.
In U.S. Pat. No. 3,700,809 to D. J. Nadon there is disclosed an inductively coupled grid cursor in which the tip of the cursor is configured in a way that is specially designed to minimize stylus tilt errors.
In U.S. Pat. No. 4,289,927 to J. L. Rogers there is disclosed graphic communciation apparatus for electronically writing information and transmitting which includes a pen having a pair of windings disposed in a predetermined angular relation with respect to each other, and a writing surface having a plurality of windings arranged substantially parallel to each other for sensing the presence of the pen from the magnetic fields of the pen coils to provide differential outputs over the writing surface with respect ot he windings of the pen apparatus, and the differential outputs being located with respect to the writing surface in an X-Y orientation.
Other known references of interest include U.S. Pat. No. 3,705,956 to M. L. Dertouzos; U.S. Pat. No. 3,735,044 to R. N. Centner et al; U.S. Pat. No. 4,368,351 to S. E. Zimmer; U.S. Pat. No. 4,375,081 to B. Blesser and U.S. Pat. No. 3,999,012 to H. Dym.
It is an object of this invention to provide a new and improved digitizing tablet system.
It is another object of this invention to provide a digitizing tablet system which automatically corrects for stylus tilt errors.
It is still another object of this invention to provide a new and improved stylus for use with a digitizing tablet system.
It is yet still another object of this invention to provide a new and improved technique for determining the position of a stylus on a work surface.